problem with sharepoint inputformtext control with updatepanel

i am using asp.net ajax  to develop an application page, i added sharepoint inputformtext controland every thing seems okay.

but after i selected dropdownlist (set autopost=true), this contol turned to be an asp rich box which has div/div .

do i need set up anything?

below is my code:

<tr>

    <td class="font_data_period">

         Email Content

                     </td>

                         <td class="font_data_period">

                            <SharePoint:InputFormTextBox RichTextMode="FullHtml" ID="RichTextEmailContent" runat="server"

                                  RichText="true" Rows="10" TextMode="MultiLine"></SharePoint:InputFormTextBox>

                              </td>

 </tr>

and this is my dropdownlist select index changing event 

<td class="font_data_period">

                                    <asp:DropDownList ID="DropDownListEmailType" runat="server" Width="380" OnSelectedIndexChanged="DropDownListEmailType_SelectedIndexChanged" AutoPostBack="true">

                                    </asp:DropDownList>

                                </td>

anybody has any idea with it?